rt2x00: rt2800lib: fix LNA_A[12] gain values for RT3593

The LNA_A[12] gain values are stored at a different
offset in the EEPROM on RT3593 based devices. However
the current code unconditionally reads those values
from the location used by other chipsets.

Fix the code to use the correct EEPROM offset.

Based on the DPO_RT5572_LinuxSTA_2.6.0.1_20120629
driver.

References:
  RT3593_EEPROM_RSSI2_OFFSET_ALNAGAIN1_24G_READ in include/chip/rt3593.h
  RT3593_EEPROM_RSSI2_OFFSET_ALNAGAIN2_5G_READ in include/chip/rt3593.h

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>